feat: triedb.Config support for arbitrary backend implementations

[ARR4N]

Why this should be merged

Allow ava-labs/coreth to use arbitrary triedb database implementations.

How this works

Introduces HashBackend and PathBackend interfaces that triedb.Database type-asserts to instead of hashdb.Database and pathdb.Backend respectively. Other interfaces are introduced to avoid having to modify {hash,path}db.Database.Reader() return values.

The explicit DBOverride field means that we don't have to modify any of the triedb constructor beyond adding a single line immediately before the return. This leaves all modifications of original files entirely mechanistic. This is, however, at the expense of compile-time guarantees of the overriding database being either a HashBackend or a PathBackend.

How this was tested

Unit test demonstrating override + plumbing.